Research On Key Physical Problems Of GaN-based Schottky Devices

Gallium nitride（GaN）is an important direct wide bandgap semiconductor material,which has been listed as one of the priority development directions in the National"14th Five-Year Plan"and the Vision 2035 Outline.GaN material has excellent physical characteristics such as wide bandgap,high electron saturation velocity and large breakdown voltage,superior to many other semiconductor materials.Among the semiconductor RF electronic devices,GaN-based devices have the highest power density,about 2～5 times higher than traditional silicon-based devices.Moreover,with outstanding performance in high frequency and high voltage fields,GaN-based RF electronic devices have been widely used in many fields such as biochemical detection,electronic warfare,satellite communication,mobile communication and digital TV.With the advantages of high switching speed,low on-resistance and high heat resistance,GaN-based power electronic devices have been widely applied in wireless charging,new energy vehicles,aerospace,laser radar and other fields since they can significantly improve the integration of circuits and systems.Despite the superior performance of GaN-based electronics devices,there are still many physical problems,especially those related to the Schottky structure,such as severe Schottky leakage current,high local temperature near the Schottky structure caused by high power density,and Schottky electrode degradation caused by long-time or high-voltage operations,restricting their further commercialization.Therefore,it is of great scientific and practical significance to study the the key physical problems related to the Schottky structure and promote the further development of GaN-based devices.Since many properties of current GaN-based electronic devices depend mainly on the Schottky structure,this thesis focuses on the important physical problems of GaN-based Schottky devices,including the reverse leakage of Schottky contact,stress degradation of Schottky gate,self-heating effect of high electron mobility transistors（HEMTs）,ion implantation and p-type doping of GaN materials.Firstly,a series of typical GaN-based Schottky devices are fabricated,including Schottky barrier diodes（SBDs）and In Al N/GaN HEMTs,etc.And several device test and characterization systems are designed and established.Secondly,the parameters of fabricated devices are measured and analyzed.The transport mechanism of Schottky contact,gate stress degradation and junction temperature measurement are investigated.Finally,in order to solve the p-type doping problem of GaN materials,GaN-based SBDs are prepared by fluorine ion（F⁺）implantation.And the electrical properties of F⁺-implanted SBDs are studied,and a model of F⁺-induced p-type doping is proposed.The main research contents of this thesis are summarized as follows.1.The temperature dependence of current-voltage（I-V）characteristics of GaN SBDs is studied.The transport mechanisms of forward current and reverse leakage current in GaN SBDs are analyzed and determined to be the thermal field emission（TFE）mechanism.A refined conduction dislocation transport model is established.It is speculated that the oxygen-related impurities may be the origin of the observed TFE leakage current.The results show that the TFE current plays a dominant role in vertical GaN SBDs,and the shallow donor levels around the dislocation core reduce the effective surface barrier,leading to the significant TFE leakage.This dislocation-related TFE model is found in good agreement with experiment.The experimental results and theoretical model can fully prove that,the observed leakage is mainly attributed to the reduction of the local conduction band energy levels near the dislocation center,rather than the continuous defect states or point defects.The reduction of the local conduction band energy levels leads to a lower effective band gap,which results in a decrease in the breakdown voltage.Compared with the previously reported researches,we conclude that oxygen-related impurities are the origin of the TFE leakage current in such GaN-based devices,which can be improved by surface passivation and compensation of N vacancies.2.The electric field distribution of the upper and lower surfaces of the barrier layer in GaN HEMTs under off state is firstly investigated,and the field strength variation with the stress is determined,which provides theoretical reference for the subsequent experimental work.The step-stress and continuous stress degradation behaviors of GaN HEMTs,as well as the hot-spot evolution rule,are studied by using a self-built micro-emission microscopic test system.The off-state degradation mechanism of GaN HEMTs is proposed.The results show that,the defect trapping effect at the In Al N barrier and In Al N/GaN heterojunction is confirmed to play an important role in the initial degradation process.When the gate leakage voltage or the stress time exceeds the critical value,the acceptor-like defects caused by the electrochemical reaction on the sample surface are responsible for the increasing gate leakage current（I_g）.The repulsion zone formed by the electrochemical reaction around hot-spots is an important factor to determine the generation location of new hot-spots.By studying the micro-emission spectroscopy of the hot-spots,it is found that the thermal shrinkage effect of bandgap is the main reason of the EL peak position red-shift.3.The self-heating effect of GaN HEMTs is studied,and the important impact of accurate junction temperature measurement on device design and reliability optimization is elucidated.At first,the junction temperature is calibrated by the Raman spectroscopy method,an authoritative calibration method in industry.Then the relationship between the maximum junction temperature(T_peak)and the dissipation power(P_diss)is determined by analyzing the temperature dependence of electrical parameters such as parasitic resistance,threshold voltage and saturated current.The results show that the method using electrical parameters are simple,but it has much lower accuracy than the Raman spectroscopy method.Thus,we propose a novel electrical method for accurately determining T_peak based on the emission microscopy technique and the temperature dependence of channel resistance.It is found when GaN HEMTs operate normally at on-state,an obvious electroluminescence（EL）phenomenon can be observed near the gate.The EL intensity exhibits an approximately triangular distribution along the channel,and the peak intensity is on the side of the gate edge near the drain.By assuming a linear relation between the EL intensity and channel temperature,a simple triangular distribution model of junction temperature is established.By measuring the relationship between the channel resistance and temperature,the temperature coefficient of triangular distribution is obtained,then the T_peak of the device is determined.From the viewpoint of the measurement principle,the junction temperature determined by the Raman spectroscopy and our new method are the real peak channel temperature,while the junction temperature obtained by electrical parameters is only the average channel temperature.Therefore,the proposed method is more suitable for evaluating the temperature characteristics of devices and can be convieniently adopted in large-scale production processes as a non-destructive measurement method.4.The existing forms of F⁺in GaN is analyzed after ion implanting.The horizontal and vertical structures of F⁺implanted GaN SBDs are prepared.The electrical characterisitics of the two structures are investigated,including I-V,C-V and EL characteristics.According to the experimental results,the mechanism of p-type doping of GaN by F⁺implantation is proposed.It can be deduced that the necessary conditions for tunneling are satisfied after ion implantation:（1）a low hole potential barrier is formed by the pinning effect of Fermi level;（2）a high electric field is formed by F^-and ionized surface donors near the surface;（3）a lot of empty states exist at the top of the valence band.Therefore,free holes can tunnel from the metal to the semiconductor and accumulate at the top of the valence band to produce p-type conductive channels.The facts that vertical I-V curves of SBDs with different sizes are completely consistent and the current amplitude is proportional to the whole wafer area,strongly confirms the formation of p-type conductive channels.In addition,the F⁺implanted MOS diode is fabricated,the existence of hole accumulation process is demonstrated,and the p-type conduction is also proved by the thermal probe method.